1. Field of the Invention
The present invention generally relates to a distance measuring apparatus for a camera and, particularly, to a multi-point distance measuring apparatus used in focus detection or the like of a camera, and, more particularly, to a multi-point distance measuring apparatus using a so-called active trigonometrical distance measuring scheme.
2. Description of the Related Art
An apparatus using a so-called active trigonometric distance measuring scheme is known as a conventional distance measuring apparatus for measuring (to be referred to as distance measurement hereinafter) a distance from a camera to an object (to be referred to as an object distance hereinafter). This conventional distance measuring apparatus is used for focus detection or the like of a camera.
A multi-point distance measuring apparatus for performing distance measurement at a plurality of points within a photographic frame is also known (e.g., Published Unexamined Japanese Patent Application No. 58-9013). By using this multi-point distance measuring apparatus, distance measurement of an object except for a target object can be prevented. For example, an in-focus state of an object except for a principal object can be prevented in focus detection of a camera.
A multi-point distance measuring apparatus using an active trigonometric distance measuring scheme will be described below.
The active trigonometric distance measuring scheme will be described first. For the sake of descriptive simplicity, an optical system for performing distance measurement of only one point within a photographic frame will be exemplified.
FIG. 7 is a view showing an arrangement of a distance measurement optical system using this active trigonometric distance measurement scheme. Referring to FIG. 7, reference numeral 1 denotes an IRED (Infrared-Emitting Diode) for intermittently emitting infrared rays; 2, a projection lens for guiding infrared rays emitted from the IRED 1 to an object O; 3, a light-receiving lens for focusing the infrared rays reflected by the object O; and 4, a so-called PSD (Position Sensitive Device) as a kind of a semiconductor optical element for receiving the infrared rays focused by the light-receiving lens 3 and outputting two different currents corresponding to light-receiving positions.
With the above arrangement, when the IRED 1 is operated to emit infrared rays, some of the emitted infrared rays are projected on the object O through the projection lens 2. Some of the infrared rays projected on the object O are reflected by the object O and are focused on the surface of the PSD 4 by the light-receiving lens 3.
A focusing position x on the surface of the PSD 4 is defined as follows: ##EQU1## where S is the distance (baseline length) between the principal points of the projection lens 2 and the light-receiving lens 3, f is the focal length of the light-receiving lens 3, and l is the object distance.
The start point of the focusing position x is an intersection between the PSD 4 and a line parallel to a line which passes through the principal point of the light-receiving lens 3 and which is obtained by connecting the emission center of the IRED 1 and the principal point of the projection lens 2.
Of the two different currents output from the PSD 4, when one current component (i.e., a component except for the other current component caused by a sunbeam and illumination light) generated by the infrared rays emitted from the IRED 1, reflected by the object O, and reaching the PSD 4, and the other current component are defined as I.sub.1 and I.sub.2, respectively, these signal currents I.sub.1 and I.sub.2 can be expressed as a function of the focusing position x a follows: ##EQU2## where I.sub.p.phi. is the total signal photocurrent, t.sub.p is the overall length of the PSD 4, and a is the distance between the start point of the focusing position x and an end portion of the PSD 4 on the IRED 1 side.
The following equation is derived from equations (1) to (3): ##EQU3## Equation (4) can be further rewritten as follows: ##EQU4## By using equation (5), the distance l to the object O can be calculated by using the signal currents I.sub.1 and I.sub.2. Equation (5) can be established by correcting a even if the position of the IRED 1 is not aligned with the optical axis of the projection lens 2.
A multi-point distance measuring apparatus using this active trigonometric distance measuring scheme will be described below. FIG. 8 is a view showing the principle of this multi-point distance measuring apparatus. Referring to FIG. 8, reference numerals 1a, 1b, and 1c denote IREDs; 2, a projection lens; and 3, a light-receiving lens; and 4a, 4b, and 4c, PSDs. Reference symbols Oa, Ob, and Oc denote objects as distance measurement target objects.
With the above arrangement, the IREDs 1a, 1b, and 1c are sequentially turned on. When the IRED 1a is turned on to emit infrared rays, signal currents output from the PSD 4a are read to perform calculations according to equation (5), thereby obtaining a distance to the object Oa. When the IRED 1b is turned on to emit infrared rays, signal currents output from the PSD 4b are read to perform calculations according to equation (5), thereby obtaining a distance to the object Ob. Similarly, when the IRED 1c is turned on to emit infrared rays, signal currents output from the PSD 4c are read to perform calculations according to equation (5), thereby obtaining a distance to the object Oc.
As described above, since equation (5) is established by properly selecting a in equation (5) even if the positions of the IREDs 1a, 1b, and 1c are not aligned on the optical axis of the projection lens 2, distance measurement (multi-point distance measurement) of a plurality of objects can be performed.
The arrangement shown in FIG. 8 uses the three IREDs 1a, 1b, and 1c as light-emitting means. However, as disclosed in the technique of Published Unexamined Japanese Patent Application No. 58-9013, one IRED may be moved to emit infrared lights at different positions.
The PSD may comprise a split type optical position detection element. In either case, multi-point distance measurement can be performed in accordance with the same principle as that of the multi-point distance measuring apparatus shown in FIG. 8.
In this multi-point distance measuring apparatus, if the size of a spot of each of infrared rays emitted from the IREDs 1a, 1b, and 1c is set to be ideally small to be a point, and each of the PSDs 4a, 4b, and 4c can receive light up to its farthest end, a minimum value (nearest distance measuring limit) l.sub.MIN (FIG. 7) of a measurable object distance is defined as follows: ##EQU5## As is apparent from equation (6), the nearest distance measuring limit l.sub.MIN is decreased as the overall length t.sub.p of each of the PSDs 4a, 4b, and 4c is increased. In other words, the nearest limit of the measurable object distance is reduced (i.e., the distance measuring precision is improved) as the overall length t.sub.p of each of the PSDs 4a, 4b, and 4c is increased.
When the overall length t.sub.p of each of the PSDs 4a, 4b, and 4c is increased, the pitch (element interval) between the PSDs 4a, 4b, and 4c is increased. For this reason, a multi-point measurable range .theta. (FIG. 8) has a larger minimum value, thus posing a problem.
More specifically, in order to reduce the minimum value of the measurable range .theta. (i.e., in order to allow multi-point distance measurement when intervals L.sub.1 and L.sub.2 between the objects Oa, Ob, and Oc are small), the pitch of the PSDs 4a, 4b, and 4c must be reduced. Therefore, the overall length t.sub.p of each of the PSDs 4a, 4b, and 4c must be reduced.
For example, assume that the focal lengths of the projection lens 2 and the light-receiving lens 3 are equal to each other. In order to set the minimum value of the measurable range .theta. to be a general value, i.e., 6.degree., the pitch of the PSDs 4a, 4b, and 4c must be set to be 1.5 mm for a focal length of 14 mm.
The overall length t.sub.p of each of the PSDs 4a, 4b, and 4c must then be set to be 1.5 mm or less, and therefore the nearest measurable limit l.sub.MIN cannot be sufficiently reduced.
In order to solve the above problem, PSDs 4a, 4b, and 4c may be disposed stepwise (oblique arrangement), as shown in FIG. 9. When the PSDs are disposed stepwise, IREDs 1a, 1b, and 1c are disposed stepwise in accordance with the oblique arrangement of the PSDs 4a, 4b, and 4c. This arrangement tends to cause that measurement positions become asymmetry at the right and left sides.
When PSDs 4a, 4b, and 4c are to be monolithically arranged, the element area is increased to cause high cost.